1. Field of the Invention
The present invention relates to a lateral shear interferometer for use in the testing of optical elements having aspheric surfaces.
2. Description of Prior Art
Optical elements are traditionally made with spherical surfaces which are easy to manufacture and test but are less precise in processing light beams. With the advance of fabrication technologies, optical elements with aspherical surfaces are now easier to made and are gradually replacing the optical elements with spherical surfaces.
Method for testing aspheric surfaces include the Form Talysurf test method, the Ronchi test method, and the interferometry test methods etc. The interferometry testing methods includes shearing interferometer, high density array interferometer, sub-Nyquist analysis method, long wavelength interferometry, two wavelength interferometry, two wavelength holography, sub-region testing method, null optics method, computer generated hologram, and zone plate interferometer etc. For a comprehensive understanding of these methods, interested readers can refer to a technical paper entitled "Interferometry Testing of Aspheric Surfaces" authored by James C. Wyant and published on SPIE Vol. 816 Interferometry Metrology (1987).
It is a drawback of conventional interferometers that the fringes in an interference pattern are in general too dense to be analyzable. A lateral shear interferometer can be used to overcome this drawback by providing low density fringes. However, the interference patterns produced by using conventional lateral shear interferometers are easily affected by external vibrations so that automated fringe analysis is impossible.